(1) Field of the Invention
The present invention relates to a mobile station apparatus and a base station apparatus for carrying out mobile communications therebetween according to TDMA technique, and more particularly, to a mobile station apparatus and a base station apparatus improved in time alignment control which is executed during signal transmission from a plurality of mobile stations to a base station.
(2) Description of the Related Art
In recent years, digital communication technique permitting high-efficiency use of radio waves via radio communication channels has been attracting attention, and there is a demand for the application of TDMA control to such communication technique in order to achieve even more efficient use of the channels.
In digital radio communication systems using the TDMA control, time alignment control is performed on signals containing a speech signal and a control signal, called hereinafter "data signals" on a physical channel for communication so as to prevent the collision of transmitted waves from substations (mobile stations) using adjacent slots of the same channel. According to the time alignment control, a guard time, if provided for the data signals to prevent the collision of transmitted waves, occupies only a short time, thus preventing the transmission efficiency from being lowered.
In conventional time alignment control, a sequence of operations is performed wherein, first, a base station determines a time alignment value based on a synchronizing burst signal transmitted from a substation and transmits the determined value to the substation, and the substation then transmits a data signal according to transmission timing adjusted based on the alignment value transmitted thereto.
FIG. 20 illustrates a conventional time alignment control sequence, or more specifically, a sequence executed at the time of channel switching. In the figure, the numbers following "S" represent step numbers in the sequence.
It is here assumed that while a substation is communicating with a first base station via a communication channel (S1), a command to switch the communication from the first to second base station is transmitted from a control station (not shown) to the first and second base stations. In response to the command, the second base station transmits a synchronizing burst signal 1 to the substation (S2), and the first base station transmits a channel designation signal specifying the channel to be switched to, to the substation (S3). After receiving the synchronizing burst signal 1 from the second base station which is associated with the channel specified by the channel designation signal, the substation transmits a synchronizing burst signal 2 to the second base station to notify the same of the reception of the synchronizing burst signal 1 (S4). Since the synchronizing burst signal 2 is not subjected to time alignment control, a guard time is set at the beginning and end of the burst signal 2. For example, in the synchronizing burst signal composed of 280 bits, the first 54 bits and the last 78 bits individually serve as a guard time.
The second base station compares the reception time of the synchronizing burst signal 2 from the substation with original reception timing thereof, and sets the derived difference as a time alignment value (S5). The second base station then transmits a synchronizing burst signal 3 carrying the time alignment value to the substation (S6). On receiving the synchronizing burst signal 3, the substation transmits a synchronizing burst signal 4 to the second base station to notify the same of the reception of the burst signal 3 (S7). This synchronizing burst signal 4 also is not subjected to the time alignment control; therefore, a guard time is provided at the beginning and end of the burst signal 4.
On receiving the synchronizing burst signal 4, the second base station transmits a data signal to the substation (S8). The substation then transmits a data signal to the second base station according to transmission timing adjusted based on the time alignment value transmitted thereto by means of the synchronizing burst signal 3 (S9).
Thus, a switching of channels is completed, and since the time alignment control is executed, the second base station can receive the transmitted signal from the substation according to the original reception timing thereof.
The synchronizing burst signals 2 and 4, which are provided with the guard times, are each transmitted from the substation to the base station in synchronism with the time of reception of a corresponding down signal from the base station. Accordingly, where the distance between the base station and the substation is great and the delay of signal transmission via a radio channel is large, correspondingly long guard times must be set.
Although the foregoing describes the case where the synchronizing burst signal is provided with guard times, a call-out signal, call-in response signal and location registration request signal, which are conventionally transmitted from a substation via a control channel, also must be provided with guard times because these signals are not subjected to the time alignment control.
Meanwhile, there is a tendency for service zones of mobile radio communications to enlarge from a zone radius of 1-2 km to 20-30 km, and greater zones bring about increased delay of data transmission and require longer guard times. From the viewpoint of transmission efficiency, however, the guard time should be as short as possible. Accordingly, there is a demand for the application of time alignment control not only to various signals transmitted from a substation via the control channel, but also to the synchronizing burst signal transmitted via the communication channel.
When switching channels during communication, the sequence of time alignment value setting, shown in FIG. 20, must be executed in order to determine the time alignment value for a new channel to which the communication is to be switched. According to this sequence, however, normal communication by means of data signals is interrupted till Step S9, causing an instantaneous disconnection of the communication. The instantaneous disconnection lowers the speech quality and thus should desirably be as short as possible.